We present photometry, spectra, and spectropolarimetry of supernova (SN)
2012ab, mostly obtained over the course of ∼300 days after discovery. SN
2012ab was a Type IIn (SN IIn) event discovered near the nucleus of spiral
galaxy 2MASXJ12224762+0536247. While its light curve resembles that of SN
1998S, its spectral evolution does not. We see indications of CSM interaction
in the strong intermediate-width emission features, the high luminosity (peak
at absolute magnitude M=−19.5), and the lack of broad absorption features in
the spectrum. The Hα emission undergoes a peculiar transition. At early
times it shows a broad blue emission wing out to −14,000 km
s−1 and a truncated red wing. Then at late times (>
100days) it shows a truncated blue wing and a very broad red emission wing
out to roughly +20,000 km s−1. This late-time broad red wing
probably arises in the reverse shock. Spectra also show an asymmetric
intermediate-width Hα component with stronger emission on the red side
at late times. The evolution of the asymmetric profiles requires a density
structure in the distant CSM that is highly aspherical. Our spectropolarimetric
data also suggest asphericity with a strong continuum polarization of ∼1−3% and depolarization in the Hα line, indicating asphericity in the
CSM at a level comparable to that in other SNe IIn. We estimate a mass-loss
rate of M˙=0.050M⊙yr−1 for vpre=100kms−1 extending back at least 75yr prior to the
SN. The strong departure from axisymmetry in the CSM of SN 2012ab may suggest
that the progenitor was an eccentric binary system undergoing eruptive mass
loss.Comment: 18 pages, 12 figure